RE: [PATCH v3 3/3] Drivers: hv: vmbus: serialize Offer and Rescind offer

[KY Srinivasan]

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx]
> Sent: Tuesday, January 20, 2015 7:45 AM
> To: KY Srinivasan; devel@xxxxxxxxxxxxxxxxxxxxxx
> Cc: Haiyang Zhang; linux-kernel@xxxxxxxxxxxxxxx; Dexuan Cui; Jason Wang;
> Radim KrÄmÃÅ; Dan Carpenter
> Subject: [PATCH v3 3/3] Drivers: hv: vmbus: serialize Offer and Rescind offer
> 
> Commit 4b2f9abea52a ("staging: hv: convert channel_mgmt.c to not call
> osd_schedule_callback")' was written under an assumption that we never
> receive Rescind offer while we're still processing the initial Offer request.
> However, the issue we fixed in 04a258c162a8 could be caused by this
> assumption not always being true.
> 
> In particular, we need to protect against the following:
> 1) Receiving a Rescind offer after we do queue_work() for processing an
> Offer
>    request and before we actually enter vmbus_process_offer(). work.func
> points
>    to vmbus_process_offer() at this moment and in vmbus_onoffer_rescind()
> we do
>    another queue_work() without a check so we'll enter
> vmbus_process_offer()
>    twice.
> 2) Receiving a Rescind offer after we enter vmbus_process_offer() and
>    especially after we set >state = CHANNEL_OPEN_STATE. Many things can
> go
>    wrong in that case, e.g. we can call free_channel() while we're still using
>    it.
> 
> Implement the required protection by changing work->func at the very end
> of
> vmbus_process_offer() and checking work->func in
> vmbus_onoffer_rescind(). In case we receive rescind offer during or before
> vmbus_process_offer() is done we set rescind flag to true and we check it at
> the end of vmbus_process_offer() so such offer will not get lost.
> 
> Suggested-by: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

Thanks Vitaly.
Signed-off-by: K. Y. Srinivasan <kys@xxxxxxxxxxxxx>
> ---
>  drivers/hv/channel_mgmt.c | 30 ++++++++++++++++++++++--------
>  1 file changed, 22 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c index
> c6fdd74..877a944 100644
> --- a/drivers/hv/channel_mgmt.c
> +++ b/drivers/hv/channel_mgmt.c
> @@ -279,9 +279,6 @@ static void vmbus_process_offer(struct work_struct
> *work)
>  	int ret;
>  	unsigned long flags;
> 
> -	/* The next possible work is rescind handling */
> -	INIT_WORK(&newchannel->work, vmbus_process_rescind_offer);
> -
>  	/* Make sure this is a new offer */
>  	spin_lock_irqsave(&vmbus_connection.channel_lock, flags);
> 
> @@ -341,7 +338,7 @@ static void vmbus_process_offer(struct work_struct
> *work)
>  			if (channel->sc_creation_callback != NULL)
>  				channel->sc_creation_callback(newchannel);
> 
> -			goto out;
> +			goto done_init_rescind;
>  		}
> 
>  		goto err_free_chan;
> @@ -382,7 +379,14 @@ static void vmbus_process_offer(struct work_struct
> *work)
>  		kfree(newchannel->device_obj);
>  		goto err_free_chan;
>  	}
> -out:
> +done_init_rescind:
> +	spin_lock_irqsave(&newchannel->lock, flags);
> +	/* The next possible work is rescind handling */
> +	INIT_WORK(&newchannel->work, vmbus_process_rescind_offer);
> +	/* Check if rescind offer was already received */
> +	if (newchannel->rescind)
> +		queue_work(newchannel->controlwq, &newchannel-
> >work);
> +	spin_unlock_irqrestore(&newchannel->lock, flags);
>  	return;
>  err_free_chan:
>  	free_channel(newchannel);
> @@ -520,6 +524,7 @@ static void vmbus_onoffer_rescind(struct
> vmbus_channel_message_header *hdr)  {
>  	struct vmbus_channel_rescind_offer *rescind;
>  	struct vmbus_channel *channel;
> +	unsigned long flags;
> 
>  	rescind = (struct vmbus_channel_rescind_offer *)hdr;
>  	channel = relid2channel(rescind->child_relid);
> @@ -528,11 +533,20 @@ static void vmbus_onoffer_rescind(struct
> vmbus_channel_message_header *hdr)
>  		/* Just return here, no channel found */
>  		return;
> 
> +	spin_lock_irqsave(&channel->lock, flags);
>  	channel->rescind = true;
> +	/*
> +	 * channel->work.func != vmbus_process_rescind_offer means we
> are still
> +	 * processing offer request and the rescind offer processing should
> be
> +	 * postponed. It will be done at the very end of
> vmbus_process_offer()
> +	 * as rescind flag is being checked there.
> +	 */
> +	if (channel->work.func == vmbus_process_rescind_offer)
> +		/* work is initialized for vmbus_process_rescind_offer() from
> +		 * vmbus_process_offer() where the channel got created */
> +		queue_work(channel->controlwq, &channel->work);
> 
> -	/* work is initialized for vmbus_process_rescind_offer() from
> -	 * vmbus_process_offer() where the channel got created */
> -	queue_work(channel->controlwq, &channel->work);
> +	spin_unlock_irqrestore(&channel->lock, flags);
>  }
> 
>  /*
> --
> 1.9.3